FIG. 18 is a block diagram of a conventional FM portable receiver. The conventional example will be described in the following with reference to the drawings.
A conventional FM portable receiver comprises (a) high frequency amplifier 1, (b) mixer 3 having one input terminal that is connected to the output of high frequency amplifier 1 and having the other input terminal that is connected to the output of local oscillator 2, (c) intermediate frequency amplifier 4 to which the output of mixer 3 is connected, (d) detector 5 to which the output of intermediate frequency amplifier 4 is connected, (e) audio amplifier 6 to which the output of detector 5 is connected, (f) jack 7 to which the output of audio amplifier 6 is connected, (g) inductor 8 connected between the ground side of jack 7 and the ground, (h) capacitor 9 connected between the ground side of jack 7 and the input of high frequency amplifier 1, (i) plug 10 inserted into the jack 7, (j) wire 11 with a length of about 1 m connected to the plug 10, and (k) earphone 12 connected to the wire 11.
The operation will be described in the following. In this FM portable receiver, the earphone 12 as an output means outputs voice. The wire 11 serves as a nearly quarter wavelength antenna for FM broadcast wave. That is, FM broadcast wave is inputted to the wire 11 that serves as an antenna. The input signal is inputted to the high frequency amplifier 1 via high-pass filter 13 comprising the inductor 8 and the capacitor 9. The FM broadcast wave inputted to the high frequency amplifier 1 is mixed by the mixer 3 with the signal from the local oscillator 2 where the frequency is varied, thereby executing channel selection.
The channel-selected signal is amplified by the intermediate frequency amplifier 4 and then detected by the detector 5 to turn into voice signal. The voice signal is amplified by the next audio amplifier 6 and outputted from the earphone 12. The high-pass filter 13 is inserted so that the FM broadcast wave is allowed to pass through but the voice is not. Due to the function of the high-pass filter 13, the wire 11 of the earphone 12 serves a dual function as an transmitter of voice signal and an antenna for FM broadcast wave.
As the documentary information of prior art related to the present invention, for example, Japanese Laid-open Patent 2002-314450 is well-known.
However, in such conventional portable receiver, since the wire 11 of earphone 12 is also used as an antenna for FM broadcast wave, there arises a problem such that the wire 11 dangles and causes unstable reception of FM broadcast wave.